Optical device with precoated underfill

ABSTRACT

A method for fabricating an optical multi-chip module (MCM) includes temporarily curing an underfill material on a chip including an optical device to prevent flow of the underfill material. The chip is flip-chip mounted on a waveguide module having a minor for directing light to or from the chip, wherein the underfill material is disposed between the chip and the waveguide module. The underfill material is cured to adhere the chip to the waveguide module.

BACKGROUND

Technical Field

The present invention relates to optical devices, and more particularlyto optical device integration using an underfill material to eliminatelosses between a photonics device and an optical component.

Description of the Related Art

An optical multi-chip module (MCM) includes optical waveguides on anorganic substrate where optical devices such as vertical cavity surfaceemitting laser (VCSEL) or photodiode (PD) chips are mounted. The lightfrom/to the device is coupled with the optical waveguides via 45-degreemirrors, where total internal reflection (TIR) minors manufactured bylaser ablation techniques are employed for high channel densityapplications.

The VCSEL/PD chips are encapsulated by an underfill when placed on theorganic substrate. During production, the underfill material entersmirror cavities located on a same side as the chips so that the TIRminors do not function properly. Loss can also be caused by aninclination of the VCSEL/PD chips when electrodes are located slightlyoff a chip center and cannot be well-controlled when they are mounted.

SUMMARY

A method for fabricating an optical multi-chip module (MCM) includestemporarily curing an underfill material on a chip including an opticaldevice to prevent flow of the underfill material. The chip is flip-chipmounted on a waveguide module having a minor for directing light to orfrom the chip, wherein the underfill material is disposed between thechip and the waveguide module. The underfill material is cured to adherethe chip to the waveguide module.

Another method for fabricating an optical multi-chip module (MCM)includes depositing an underfill material over a wafer having aplurality of chips with raised electrodes, the plurality of chipsincluding optical devices; removing the underfill material from theraised electrodes; temporarily curing the underfill material; dicing thewafer to separate the plurality of chips; flip-chip mounting a chip ofthe plurality of chips on a waveguide module having a minor fordirecting light to or from the chip, wherein the underfill material isdisposed between the chip and the waveguide module; and curing theunderfill material to adhere the chip to the waveguide module.

An optical multi-chip module (MCM) includes a waveguide module having acavity with a mirror. The minor is configured to direct light into orfrom a waveguide formed in the waveguide module. A chip flip-chip ismounted on the waveguide module to have a light input or output formedon the chip aligned with the minor. A reflowable underfill material isdisposed between the chip and the waveguide module to adhere the chip tothe waveguide module without filling the cavity.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The disclosure will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a side view of a wafer covered by an underfill material andhaving optical devices, such as, vertical cavity surface emitting laser(VCSEL) or photodiode (PD) chips formed therein with raised electrodesin accordance with the present principles;

FIG. 2 is a side view of the wafer of FIG. 1 showing the underfillmaterial of the wafer being processed with a mask during a lithographyprocess to remove the underfill material from the raised electrodes inaccordance with the present principles;

FIG. 3 is a side view of the wafer of FIG. 2 showing the underfillmaterial removed from the raised electrodes and temporarily cured inaccordance with the present principles;

FIG. 4 is a side view of a chip diced from the wafer of FIG. 3 inaccordance with the present principles;

FIG. 5 is a cross-sectional view of an optical multi-chip module (MCM)including a cured underfill material adhering the chip of FIG. 4 to awaveguide module without interfering with a light cavity in thewaveguide module in accordance with the present principles; and

FIG. 6 is a block/flow diagram showing a method for forming an opticalmulti-chip module in accordance with illustrative embodiments.

DETAILED DESCRIPTION

In accordance with the present principles, structures and methods areprovided for optical devices that are employed with infrared-transparentphotosensitive thermal-curing underfill. The underfill may include,e.g., cyclotene resin, and may be pre-coated on optical chips orprovided on optical waveguide-integrated organic substrates with totalinternal reflection (TIR) mirrors. Since the underfill is pre-coated andsemi-cured in advance, there is no danger of filling mirror cavities. Inaddition, an optical path between the optical device and a waveguide isstill filled with the underfill to eliminate any air gaps. The TIR minorcavities remain open on the waveguides at a side where the devices aremounted, and are not filled with the underfill to maintain low-lossoptical coupling. Use of transparent underfill fills between verticalcavity surface emitting laser (VCSEL) or photodiode (PD) (VCSEL/PD)chips and the waveguide, still results in loss reduction by eliminatingair interface reflection in the gap space. In accordance with thepresent principles more precise control of the underfill area isprovided.

A distance between the devices and the waveguides is minimized forlow-loss optical coupling by removing the underfill at an electrode areaof the device and using through-waveguide-vias instead of insertingflexible printed circuits between those two components for electricconnection of the devices. The underfill thickness is also minimized,and no lens is employed on the optical device to further reduce cost.

It is to be understood that the present invention will be described interms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps may be varied within the scope of the present invention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements may also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements may be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

The present embodiments may include a design for an integrated circuitchip, which may be created in a graphical computer programming language,and stored in a computer storage medium (such as a disk, tape, physicalhard drive, or virtual hard drive such as in a storage access network).If the designer does not fabricate chips or the photolithographic masksused to fabricate chips, the designer may transmit the resulting designby physical means (e.g., by providing a copy of the storage mediumstoring the design) or electronically (e.g., through the Internet) tosuch entities, directly or indirectly. The stored design is thenconverted into the appropriate format (e.g., GDSII) for the fabricationof photolithographic masks, which typically include multiple copies ofthe chip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein may be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as an organiccarrier or a ceramic carrier that has either or both surfaceinterconnections or buried interconnections). In any case the chip isthen integrated with other chips, discrete circuit elements, and/orother signal processing devices as part of either (a) an intermediateproduct, such as a motherboard, or (b) an end product. The end productcan be any product that includes integrated circuit chips, ranging fromtoys and other low-end applications to advanced computer products havinga display, a keyboard or other input device, and a central processor.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present principles, as well as other variations thereof, means thata particular feature, structure, characteristic, and so forth describedin connection with the embodiment is included in at least one embodimentof the present principles. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This may be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

Referring now to the drawings in which like numerals represent the sameor similar elements and initially to FIG. 1, a wafer 10 is shown forprocessing in accordance with one illustrative embodiment. The wafer 10includes a plurality of chips 15 formed thereon. The chips 15 mayinclude lasers, photodiodes or other light emitting or receivingdevices, herein referred to as optical devices. In one embodiment, thechips 15 include vertical cavity surface emitting laser (VCSEL) chips orphotodiode (PD) chips. The chips 15 include one or more pads or contacts14 for making electrical connections to the chip 15. The contacts 14 maybe deposited on the wafer 10 and patterned to shape the contacts 14. Thedeposition of the contacts 14 may include any process for forming metalincluding, e.g., sputtering, evaporation, chemical vapor deposition,etc. The contacts 14 may include a solder ball 16 formed thereon formaking an electrical connection to other components as will bedescribed. The contacts 14 and solder balls 16 will be collectivelyreferred to as electrodes 25.

An underfill material or underfill 18 is formed over the wafer 10. Theunderfill 18 may include, e.g., cyclotene resin. The underfill 18 may beapplied using a spin on process, although other processes may beemployed to apply the underfill 18 on the wafer. The underfill thicknessis adjustable by controlling a rotation speed of a spin coater. Thethickness of the underfill 18 is provided in accordance with a gapdistance needed between a chip and a waveguide.

Referring to FIG. 2, after forming the underfill 18, a soft bake processmay be performed to harden the underfill 18. In one embodiment, thewafer 10 with the underfill 18 is baked at a temperature of betweenabout 60 degrees C. and about 80 degrees C. for about 90 seconds. Theunderfill 18 over the electrodes 25 is removed by a photolithographyprocess. The photolithography process may include positioning a mask 20over the wafer 10, exposing the underfill 18 to light in accordance withthe mask 20 to cause cross-linking, and developing the underfill layer18 to remove portions of the underfill 18 over the electrodes 25.

Referring to FIG. 3, the underfill 18 is subjected to a temporaryhardening process by subjecting the underfill 18 to a reflow at between100 degrees C. to about 140 degrees C., and preferably about 125 degreeC. for about 3 minutes. The underfill 18 is allowed to cool.

Referring to FIG. 4, the wafer 10 is diced into individual chips 40.Each chip 40 includes its own electrodes 25, which will be employed inmaking electrical connections to other components.

Referring to FIG. 5, a flip-chip assembly is performed to couple thechip 40 to a waveguide 44 formed on a substrate 42 to form an opticalmulti-chip module (MCM) 100. The substrate 42 may include an organicmaterial. The waveguide 44 is formed between cladding layers 48. Acontact connection 50 receives the electrode 25 therein and includesconductive material to create electrical connections to the chip 40. Alight emitting or receiving region 52 on the chip 40 is aligned with amirror 45. The minor 45 such as a total internal reflection (TIR) mirroris employed to redirect light into the waveguide 44 from the lightemitting or receiving region 52 of the chip 40. A cavity 46 for theminor 45 may be formed by laser ablation to be at a 45 degree anglerelative to the surface of the chip 40.

There are a number of reasons and advantages to having the mirror 45 onthe mounting side of the chip 40. For example, the mirror 45 is providedin close proximity to the light emission position of the chip 40. Also,the chip 40 may be employed to cover the cavity 46 for the mirror 45.

Once the chip 40 is positioned, a curing process is performed, which mayinclude adhesion of the underfill 18 by a reflow process. The reflowprocess may include subjecting the underfill to a temperature of betweenabout 130 degrees C. to about 160 degrees C., preferably about 150degrees C. for about 5 minutes. Then, a hardening process is performedat a temperature of between about 200 degrees C. and about 220 degreesC., preferably about 210 degrees C. for about 40 minutes. Solder bondingof the electrode 25 to the contact connection is performed by a 240degrees C. to about 260 degrees C. solder reflow process.

By pre-coating the chip 40 with the underfill 18, the underfill 18remains well-controlled. A thickness of the underfill 18 is controlledat its deposition and forms a highly controlled gap dimension 54. Inaddition, reflowing the underfill 18 does not permit the underfillmaterial 18 to flow and fill in the cavity 46. Instead, the underfill 18is softened and provides adhesion without the ill-effects ofconventional devices, which fill the cavity and result in opticallosses.

Since the underfill is pre-coated and cured in advance, there is nodanger of filling mirror cavities. Air gaps between the chip 40 and thetop cladding layer 48 are virtually eliminated between the chip 40 andthe minor 45. The present principles provide a low cost solution with ayield improvement for optical MCM manufacturing by preventing underfill18 from entering TIR mirror cavities 46. The pre-coating with underfill18 is concurrently provided on multiple optical devices by wafer-levelprocessing.

The optical MCM 100 provides low power consumption (e.g., opticalcoupling loss reduction) since distance between the chip 40 and thewaveguide 44 is reduced by underfill thickness control. In oneembodiment, gap distances 54 of less than about 5 microns are provided.In addition, by including a uniform thickness of underfill 18,inclination of the chip 40 relative a waveguide module 60 is reduced oreliminated. In one embodiment, the chip 40 and waveguide cladding 48surfaces are stuck together flat with the underfill 18 acting as anadhesive. This also improves the interfaces (and therefore lighttransmission) through the underfill 18.

While one embodiment depicts applying the underfill material to a wafer,other embodiments may apply the underfill material directly to a chipinstead of the wafer. Processing is similar but employs one or morechips instead of the wafer.

Experiment: Underfill (18) was dispensed and cured on polymer waveguidesfor confirming adhesion and evaluating optical properties. In oneprocess, a pre-coat of underfill on a glass substrate by a spin process(rotational speed of spin coater being 3000 rotations per minute (rpm))was performed. A soft bake at 70 degrees C. for 90 seconds and atemporary cure at 125 degrees C. for 3 minutes were performed. Then, awaveguide-integrated substrate on glass was attached and fixed using aclip. An underfill adhesion includes 150 degrees C. for 5 minutes, and acure at 210 degrees C. for 40 minutes.

Insertion loss measurement results were taken for the following twoconfigurations. The first configuration included a waveguide and a glasssubstrate with index matching fluid (IMF) dispensed between a waveguideand a glass. The loss measured by a photodetector (1 cm) was −0.46 dB.The second configuration included a waveguide with underfill inaccordance with the present principles. The loss measured by aphotodetector (1 cm) was −0.40 dB with no insertion loss degradationafter underfill cure (the 0.06 dB improvement is within the measurementerror range). This means that no air layer existed between the waveguideand underfill.

In addition, the present principles do not require a lens or flexibleprinted circuit (FPC) between the chip 40 and the cavity 46. While nolens or FPC is needed, these components may be employed in someembodiments depending on the application. However, these components addcosts and complexity to the manufacturing process.

Referring to FIG. 6, a method for fabricating an optical multi-chipmodule (MCM) is shown in accordance with illustrative embodiments. Insome alternative implementations, the functions noted in the blocks mayoccur out of the order noted in the figures. For example, two blocksshown in succession may, in fact, be executed substantiallyconcurrently, or the blocks may sometimes be executed in the reverseorder, depending upon the functionality involved. It will also be notedthat each block of the block diagrams and/or flowchart illustration, andcombinations of blocks in the block diagrams and/or flowchartillustration, can be implemented by special purpose hardware-basedsystems that perform the specified functions or acts or carry outcombinations of special purpose hardware and computer instructions.

In block 202, an underfill material is applied to a wafer or chip. Thismay include a spin-on process or other deposition process. Theapplication of the underfill material is controlled to control a gapdistance. The wafer includes chips. The chip or chips include opticaldevices, such as photodiodes and/or lasers. The underfill materialincludes a semi-curable and reflowable resin. Semi-curable refers to amaterial that is stable being partially (temporarily or semi-) cured andthen can be fully cured at a later point. Reflowable refers to thematerial as being capable of melting and re-solidifying. In oneembodiment, the underfill material includes a cyclotene resin.

In block 204, a soft bake may be performed to provide some structure tothe underfill material. The soft bake may include, e.g., heating at 70degrees C. for about 90 seconds. In block 206, the underfill materialmay be removed from raised electrodes formed on the wafer or chip. Thismay include a lithography process to expose the underfill materialthrough a mask and remove/develop uncross-linked portions to expose theraised electrodes.

In block 208, the underfill material is temporarily cured on the waferor chip to prevent flow of the underfill material. Temporary curing theunderfill material may include hardening the underfill material atbetween 100 degrees C. to about 140 degrees C.

In block 210, if a wafer was employed, the wafer is diced or separatedinto chips. In block 212, the chip is flip-chip mounted on a waveguidemodule. The light emitting or receiving chip is aligned with a mirrorfor directing light from/to the chip. The underfill material is disposedbetween the chip and the waveguide module. Since the chip is pre-coatedwith a solid or hardened underfill material, the underfill material doesnot interfere with a cavity in the waveguide module by flowing into thecavity and causing light attenuation. The cavity may include a mirrorand a waveguide depending on the design.

In block 214, the underfill material is fully cured to adhere the chipto the waveguide module. The full cure may include reflowing theunderfill material at a temperature of between about 130 degrees C. toabout 160 degrees C. to provide adhesion in block 216, and hardening theunderfill material at a temperature of between about 200 degrees C. andabout 220 degrees C. in block 218.

In block 220, solder joints are reflowed to make solder connectionsbetween the light emitting or receiving chip and the waveguide module.In block 222, processing may continue to complete the device/module.

Having described preferred embodiments for optical device with precoatedunderfill (which are intended to be illustrative and not limiting), itis noted that modifications and variations can be made by personsskilled in the art in light of the above teachings. It is therefore tobe understood that changes may be made in the particular embodimentsdisclosed which are within the scope of the invention as outlined by theappended claims. Having thus described aspects of the invention, withthe details and particularity required by the patent laws, what isclaimed and desired protected by Letters Patent is set forth in theappended claims.

1. A method for fabricating an optical multi-chip module (MCM),comprising: temporarily curing an underfill material on a chip includingat least one optical device to prevent flow of the underfill materialinto a mirror cavity; flip-chip mounting the chip on a waveguide modulehaving a mirror for directing light to or from the chip, wherein theunderfill material forms a gap, determined by the temporarily curedunderfill material, between the chip and the waveguide module; andcuring the underfill material to adhere the chip to the waveguidemodule, wherein the mirror cavity remains free of underfill material. 2.The method as recited in claim 1, wherein the underfill materialincludes a semi-curable and reflowable resin.
 3. The method as recitedin claim 1, wherein the underfill material includes a resin.
 4. Themethod as recited in claim 1, wherein flip-chip mounting the chipincludes soldering the chip to the waveguide module.
 5. The method asrecited in claim 1, wherein temporarily curing the underfill materialincludes hardening the underfill material at between 100 degrees C. toabout 140 degrees C.
 6. The method as recited in claim 1, furthercomprising spinning on the underfill material and removing the underfillmaterial from chip electrodes.
 7. The method as recited in claim 1,wherein curing the underfill material includes: reflowing the underfillmaterial at a temperature of between about 130 degrees C. to about 160degrees C. to provide adhesion; and hardening the underfill material ata temperature of between about 200 degrees C. and about 220 degrees C.8. A method for fabricating an optical multi-chip module (MCM),comprising: depositing an underfill material over a wafer having aplurality of chips with raised electrodes, the plurality of chipsincluding optical devices; removing the underfill material from theraised electrodes; temporarily curing the underfill material; dicing thewafer to separate the plurality of chips; flip-chip mounting the chip ona waveguide module having a mirror for directing light to or from thechip, wherein the underfill material forms a gap, determined by thetemporarily cured underfill material, between the chip and the waveguidemodule; and curing the underfill material to adhere the chip to thewaveguide module, wherein the mirror cavity remains free of underfillmaterial.
 9. The method as recited in claim 8, wherein the underfillmaterial includes a semi-curable and reflowable resin.
 10. The method asrecited in claim 8, wherein the underfill material includes a resin. 11.The method as recited in claim 8, wherein flip-chip mounting the chipincludes soldering the chip to the waveguide module.
 12. The method asrecited in claim 8, wherein temporarily curing the underfill materialincludes hardening the underfill material at between 100 degrees C. toabout 140 degrees C.
 13. The method as recited in claim 8, whereindepositing the underfill material includes spinning on the underfillmaterial.
 14. The method as recited in claim 13, further comprisingcontrolling a thickness of the underfill material by controlling aspinning speed.
 15. The method as recited in claim 8, wherein curing theunderfill material includes: reflowing the underfill material at atemperature of between about 130 degrees C. to about 160 degrees C. toprovide adhesion; and hardening the underfill material at a temperatureof between about 200 degrees C. and about 220 degrees C.
 16. The methodas recited in claim 8, wherein removing the underfill material from theraised electrodes includes employing lithography to develop and removethe underfill material from the raised electrodes. 17.-20. (canceled)